Photocathode devices are optoelectronic detectors which use the photoemissive effect to detect light energy. Thus, when photons impinge the surface of a photocathode device, the impinging photons cause electrons to be emitted therefrom. Many photocathode devices are made from semiconductor materials such as gallium arsenide (GaAs) which exhibit the photoemissive effect. While GaAs is preferred, it is noted that other III-V materials can be used such as GaP, GaInAsP, InAsP and so on. In a semiconductor photocathode device, photons are absorbed by a photoemissive semiconductor material. The absorbed photons cause the carrier density of the semiconductor material to increase, thereby causing the material to generate a photocurrent.
Semiconductor photocathode structures are employed in the image intensifiers of state of the art night vision devices. These photocathode structures typically use a semiconductor epilayer for the photon absorbing material. The semiconductor epilayer is thermally and mechanically bonded to a glass faceplate of the image intensifier to provide a rigid, vacuum supporting tube structure. The peripheral surface of both the semiconductor epilayer and the glass faceplate are coated with a conducting material, such as chrome, to provide an electrical contact to the photocathode semiconductor structure. Typically in such photocathode structures, the common cathode material is p-type GaAs. However, the chrome contact layer forms a poor ohmic contact at the low p-type doping concentrations of the GaAs common cathode material. In any case, the chrome contact layer is usually deposited prior to the cathode structure being placed in a final etch solution which is used to prepare the cathode for entry into an ultra-high vacuum station. Consequently, the final etch process removes only the uppermost layer of the semiconducting material using the chrome contact layer as an etch mask.
The thickness of the epilayer causes a large discontinuity in height between the epilayer and the faceplate which the conductive contact layer must be contoured to. Covering such a large vertical step with a thin layer of material often leads to gaps in the coverage of the material resulting in an incomplete contact which causes substantially higher operating voltages. Also contributing to substantially higher operating voltages is the poor ohmic contact quality provided by using a chrome contact layer with a low concentration p-type doped GaAs common cathode material. Moreover, the thin contact layer is easily damaged by physical and mechanical handling operations which leads to peeling of the conductive layer. When a high voltage is applied between the cathode and the input of a microchannel plate of an image intensifier, the peeling layer leads to undesirable emission points. Since the conductive layer is closer to the microchannel plate input than the emission surface of the photocathode, any contaminates on the conductive layer can lead to further undesirable emission points.
Accordingly, there is a need for a semiconductor photocathode structure that substantially overcomes the problem of undesirable emission points and excessive fragility during handling operations.